1. Field of the Invention
The present invention relates to a coupling apparatus provided with double clutches which is connected between first and second rotary shafts to transmit power from the first shaft to the second shaft or vice versa, and in particular suitable for use with a differential gear assembly mounted on an automotive vehicle to limit differential motion thereof as conditions require.
2. Description of the Prior Art
A limited-slip differential gear assembly provided with a single multidisk clutch for limiting differential operation is disclosed in U.S. Pat. No. 4,718,303 or U.S. Pat. No. 4,781,078. In each of these prior-art differential gear assemblies provided with a single multidisk clutch, the engagement force of the multidisk clutch is controlled by an electromagnet. That is, when the electromagnet is energized, friction force is generated between two contacting friction plates due to the differential motion to actuate a cam mechanism, so that the friction plates are further brought into tight contact with each other by a cam thrust to increase the friction force into the multidisk clutch engagement.
In this prior-art coupling apparatus, however, when the friction plates of the multidisk clutch wear away, since the air gap between the electromagnet and a plunger member varies and therefore the characteristics between magnet current value and clutch engagement force change, there exists a problem in that an appropriate differential limiting operation will not be controlled stably and reliably by the electromagnet. Further, where the thrust force is increased by changing the cam angle, since the clutch engagement force multiplied by the cam increases greatly with increasing frictional coefficient .mu. of the friction plates, as shown by the prior-art curve shown in FIG. 1(C), it is impossible to stably control a large differential limitation force by the cam. Therefore, where a large torque is required to be controlled, the diameter of the clutch inevitably increases and therefore the size of the coupling apparatus also increases.